The increase in prison population coupled with the shortage of adequate prison space has increased the interest in using personnel monitoring devices as part of a home arrest system.
Various personnel monitoring devices have been proposed in the past. For example, U.S. Pat. No. 4,973,944 discloses a proximity device which is strapped by a band to a person's leg or wrist. The device transmits encoded RF signals which are received by a receiving station. When the device is taken out of range or the band is severed, the receiving station does not receive the transmitted signals, and an alert condition is initiated. The band is made of plastic and has electrically conductive material adhered to its outer surface such that a resistive DC circuit path exists when both ends of the band are brought into contact with one another. Likewise, U.S. Pat. No. 4,980,671 discloses a transmitter secured to the body of a person by a conductive mounting strap. An electronic latch outputs a signal indicative of whether or not the strap has been broken.
These known personnel monitoring devices have the disadvantage that conductive elements are employed to maintain continuity through the strap. However, conductive elements may be effectively bypassed through the use of appropriate jumpers. Thus, one can remove the strap while bypassing the conductive element, thereby rendering the device ineffective for its intended purpose.
Fiber optic technology has been used as a substitute for conductive elements in certain theft detection devices. For example, U.S. Pat. No. 5,055,827 discloses an optical fiber attached to an appliance and connected to a control box. An alarm is activated if the detected light signal is attenuated, such as would occur if the optical fiber were bent or broken in an attempt to steal the appliance. However, no one has heretofore employed fiber optic technology in a personnel monitoring device to provide a signal indicative of a tamper condition.
According to the present invention, fiber optic technology is utilized in a personnel monitoring device to avoid the disadvantage described above.